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Determination of the ion temperature in matter exposed to intense ultrashort laser pulses

Modern femtosecond visible lasers and free electron lasers in the extended ultraviolet (EUV) energy range (such as Fermi@Elettra) can deliver monochromatic pulsed radiation (Δt ~ 100 fs) with more than 1012 photons/pulse. Such intense beams combined with suitable optical systems allow to achieve  outstanding irradiances greater than 1012 W/cm2 within a limited region smaller than 10 μm. When condensed matter is exposed to light with this level of intensity, it is suddenly heated (T > 10000 K) and compressed (p > 100 kbar).

This gives to researchers the opportunity to generate in the laboratory unexplored states of matter present, in stationary thermodynamic conditions, only in the astrophysical context such as in the interior of large planets or small stars. Unfortunately, laser-generated excited states of matter exhibit typical lifetimes of several picoseconds. As a consequence, one needs ultrafast experimental diagnostic probes capable to monitor transient physical properties of the sample with temporal and spatial resolution respectively of few hundreds of femtoseconds and few microns. These constraints make this class of measurements particularly sophisticated and limit our ability to determine the thermodynamic state of the sample during experiments. For example, the estimation of the ion temperature (Ti) in matter heated by pulsed lasers is still an open issue.


Figure 1:  Simulated temperature profile on the rear side of a stainless steel foil (thickness 5 μm) after absorption of a laser pulse of about 2 mJ focused on the front side of the same foil to form a Gaussian spot of about 70 μm (FWHM). ΔT is shown as a function of time and radial distance from the center of the laser axis.

Figure 2: Values of the stainless steel sample ion temperature (Ti) as a function of the laser pulse energy obtained by fitting theoretical model functions with the experimental voltage signal (proportional to the temperature) provided by an infrared pyrometer.

Our numerical simulations (Fig. 1) have confirmed that when a thin metallic sample is exposed to an intense sub-picosecond laser pulse on its front side, a slow temperature fluctuation (ΔT < 100 K , Δt ~1 ms) occurs on the rear cold side. Moreover, we have demonstrated that the temporal profile (ΔT(t)) of that fluctuation is governed by the classical heat diffusion law. This enable us to calculate the amount of energy deposited onto the sample lattice by the laser pulse and finally retrieve the value of Ti, provided an accurate measurement of ΔT(t) on the cold rear side of the sample. In order to measure ΔT(t), avoiding direct contact with the sample, we have developed a special pyrometer sensitive to the infrared radiation emitted by the rear side of the sample. The pyrometer has been designed to monitor a circular region of diameter 250 μm centered on the trajectory of the laser.
Preliminary measurements have been carried out on a stainless steel foil (thickness 5 μm) excited by a Ti:Sa laser (wavelength 800 nm, pulse length 200 fs).

We have discovered that a single laser pulse of 2.9 mJ focused to form a Gaussian spot of 70 μm (FWHM) deposits only a small fraction of its energy (~33 μJ) on the stainless steel ion lattice. However, our simulations assess that this is enough to raise the local ion temperature up to Ti = 8300 K (~ 0.72 eV, see Fig. 2).
Our method can be used with both conventional table-top femtosecond lasers and innovative EUV free electron lasers. The capability of measuring Ti in ultrafast-heated matter can accelerate further advances in research on unexplored states of matter. Moreover, our approach can make a tangible impact on critical technological fields such as the production of energy by means of the inertial confinement fusion process.


This research was conducted by the research of the EIS-TIMEX beamline at Fermi@Elettra laboratory, in collaboration with researchers of the Physics Department, University of L’Aquila and of the Physics Department, University of Camerino.

  • Emiliano Principi, Riccardo Cucini, Alessandro Gessini, Filippo Bencivenga, Francesco D’Amico and Claudio Masciovecchio, Sincrotrone Trieste S.C.p.A., Trieste, Italy
  • Adriano Filipponi, Dipartimento di Scienze Fisiche e Chimiche, University of L’Aquila, L’Aquila, Italy
  • Andrea Di Cicco, CNISM, Dipartimento di Fisica, University of Camerino, Camerino (MC), Italy and IMPMC-CNRS UMR 7590, University P. et M. Curie, Paris, France

Reference

E. Principi, R. Cucini, A. Filipponi, A. Gessini, F. Bencivenga, F. D'Amico, A. Di Cicco, and C. Masciovecchio, Phys. Rev. Lett. 109, 025005 (2012); DOI:10.1103/PhysRevLett.109.025005

Last Updated on Thursday, 04 October 2012 16:26